The present invention relates to a functionalised polymer, a method of producing same and curable compositions containing same.
Conventional curable compositions, in particular those susceptible to curing by exposure to radiation such as UV light, contain functionalised oligomeric species. Such oligomeric species tend to be based on acrylated-epoxy, acrylated-polyester or acrylated-urethane resins and to have molecular weights (number average) of from 500 to 2000. The composition is cured by the reaction of the acrylate functional groups.
Additionally, such conventional curable compositions may also contain reactive monomeric species such as isobornyl acrylate, hexanediol diacrylate, tripropylene glycol diacrylate and trimethylol propane triacrylate.
These conventional curable compositions often shrink during curing. However, this tendency may be combated by the inclusion of a relatively high molecular weight (number average) copolymer, i.e. greater than 2000. The copolymer is present as a solute and the oligomeric species and (when present) reactive monomeric species act as a solvent. The copolymer is typically a random copolymer formed from two or more of methyl methacrylate, n-butyl methacrylate, ethyl methacrylate and methacrylic acid monomers.
In order to further improve the properties of the curable composition, it would be very desirable to cross-link the copolymer with the functional, e.g. acrylate, groups of the oligomeric species and (when present) reactive monomeric species during curing. However, such cross-linking would require the copolymer to have pendant reactive groups of similar structure to the functional groups of the oligomeric species and also of the reactive monomeric species. This would be difficult to achieve by using multifunctional monomers to make the functional copolymer because pendant reactive groups present on the monomers, which would participate in such a cross-linking reaction, would also participate in the polymerisation process to provide a prematurely cross-linked structure. In order to overcome this problem a copolymer with a suitable pendant group could be functionalised by esterification at 80-100 deg C for example as described in CHEMISTRY and TECHNOLOGY OF UV and EB FORMULATION FOR COATINGS. INKS and PAINTS, VOLUME 2. Prepolymers and Reactive Diluents for UV and EB Curable Formulations, N. S. Allen et al, page 124xe2x80x94Formation of a Polyester Acrylate. However at the reaction temperature for esterification the functionalised pendant reactive groups may react to form a prematurely cross-linked structure.
It is also desirable to be able to functionalise the copolymer in a solvent which is the functionalised oligomeric species and possibly the reactive monomeric species but this is currently not possible because at such high esterification temperatures the oligomeric and monomeric species would be likely to polymerise and react with the functionalised copolymer leading to premature polymerisation and cross-linking. Thus far no practical method of preparing such a functionalised copolymer has been identified.
It is an object of the present invention to provide such a functionalised copolymer, a new method for its preparation and a curable composition containing such a functionalised copolymer.
Accordingly in a first aspect the present invention provides a copolymer of general formula (I) 
wherein A is a residue of at least one (meth)acrylate monomeric species, B is a residue of at least one monomeric species copolymerisable with A, R is a (meth)acrylate functionalised pendant ester group, X1 and X2 are terminal groups which may be the same or different, n and p are both at least one and are chosen such that the copolymer has a number average molecular weight of more than 2000.
In a second aspect, the present invention provides a process for the preparation of a copolymer (I) 
wherein A is a residue of at least one (meth)acrylate monomeric species, B is a residue of at least one monomeric species copolymerisable with A. R is a (meth)acrylate functionalised pendant ester group, X1 and X2 are terminal groups which may be the same or different, n and p are both at least one and are chosen such that the copolymer has a number average molecular weight of more than 2000 which process comprises the steps of
(i) polymerising by free radical polymerisation the at least one (meth)acrylate monomeric species providing residue A with the monomeric species providing residue B to form a copolymer of general formula (II) 
xe2x80x83wherein A, B, X1, X2, n and p are as defined above and Y is a pendant reactive group capable of undergoing esterification;
(ii) dissolving the copolymer (II) in a solvent; and thereafter
(iii) esterifying the pendant group Y with a moiety that carries both a reactive group Q capable of undergoing esterification with the group Y and also a (meth)acrylate group so as to form groups R in the presence of a reagent of the general formula (III)
Z1xe2x88x92N=C=Nxe2x88x92Z2xe2x80x83xe2x80x83(III)
xe2x80x83wherein Z1 and Z2 are end groups which are capable of assisting in the solubilisation of the reagent by the solvent.
In a third aspect, the present invention provides a curable composition which comprises a solution of a copolymer of general formula (I) 
wherein A is a residue of at least one (meth)acrylate monomeric species, B is a residue of at least one monomeric species copolymerisable with A, R is a (meth)acrylate functionalised pendant ester group, X1 and X2 are terminal groups which may be the same or different, n and p are both at least one and are chosen such that the copolymer has a number average molecular weight of more than 2000 in a solvent, which solvent is a functionalised oligomeric species having number average molecular weight of 2000 or less and/or a reactive monomeric species.
In a fourth aspect, the present invention provides a coating formed by curing a curable composition comprising a solution of a copolymer of general formula (I) 
wherein A is a residue of at least one (meth)acrylate monomeric species, B is a residue of at least one monomeric species copolymerisable with A, R is a (meth)acrylate functionalised pendant ester group, X1 and X2 are terminal groups which may be the same or different, n and p are both at least one and are chosen such that the copolymer has a number average molecular weight of more than 2000 in a solvent, which solvent is a cross-linkable oligomeric species and/or reactive monomeric species having number average molecular weight of less than 2000.
Suitable (meth)acrylate monomeric species which can provide residues A include lower alkyl, i.e. C1 to C20 alkyl, (meth)acrylates, e.g. methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, iso-butyl methacrylate, t-butyl methacrylate, 2-ethyl hexyl methacrylate, octyl methacrylate, ethyl acrylate, butyl acrylate. Additionally, cyclic alkyl monomeric species may be used such as cyclohexyl methacrylate and isobornyl methacrylate. Preferably, mixtures of such monomeric species are used in order to optimise the characteristics of the copolymer. In particular, combinations of methyl methacrylate with one or more of n-butyl methacrylate and ethyl methacrylate may be used to good effect.
Suitable monomeric species which can provide residues B include hydroxy alkyl (meth)acrylates such as hydroxyethyl methacrylate (HEMA), hydroxypropyl methacrylate, hydroxybutyl methacrylate, hydroxyethyl acrylate, hydroxypropyl acrylate and hydroxybutyl acrylate; acid containing monomers such as methacrylic acid and acrylic acid; and amine containing monomers such as amino ethyl methacrylate and amino ethyl acrylate.
A and B may be the same or different.
The terminal groups X1 and X2 are determined by the monomeric species used and also the reagents used in the free radical polymerisation of the monomers. Additionally, it may be possible to further functionalise the copolymer so that one or other or both are a group R.
The group R is a (meth)acrylate functionalised pendant ester group. Preferably the (meth)acrylate functionality is terminal to the group R. The group R is formed by the esterification of a pendant reactive group Y with a reactive group Q carried by a (meth)acrylate functionalised moiety. The group Y may be hydroxyl, carboxylic acid, amino (primary or secondary), mercaptan or amide with the reactive group Q being selected appropriately such that a condensation reaction results. Preferably, group Y and the reactive group Q are hydroxyl or carboxyl, where Q and Y must be different.
The parameters n and p are both at least 1 and are chosen such that the number average molecular weight of the copolymer is more than 2000, preferably from 2000 to 100000 and particularly from 3000 to 40000. Preferably the parameters n and p are chosen such that the ratio of n:p is from 1:1 to 500:1, particularly from 3:1 to 100:1 and especially from 3:1 to 50:1. Where the at least (meth)acrylate monomeric species is a mixture of monomers then for each monomer there will be an individual value of n and the sum of the individual values of n should be used in the foregoing relationships between n and p. Similar allowances should be made where a mixture of monomeric species provide the residues B.
Preferably the copolymer (II) is prepared using conventional free radical polymerisation techniques such as those used in the suspension, solution, emulsion and bulk polymerisation of (meth)acrylate polymers. The copolymer may be a block, random or alternating copolymer.
In the process of the present invention the copolymer (II) is functionalised by first dissolving the copolymer (II) in a suitable solvent. Examples of suitable solvents include aromatic hydrocarbons such as toluene and xylene; ketones such as acetone and methyl ethyl ketone; esters such as ethyl acetate, propyl acetate and butyl acetate; ethers such as tetrahydrofuran; and chlorinated hydrocarbons such as chloroform and dichloromethane. Suitable solvents also include reactive monomers which constitute components of conventional curable compositions e.g. isobornyl acrylate, hexanediol diacrylate, tripropylene glycol diacrylate and trimethylolpropane triacrylate. Additionally, other mono, di, tri and tetra functional reactive monomers may be used such as butyl acrylate, 2-ethyl hexylacrylate, octyl acrylate, phenoxy ethyl acrylate, propylene glycol acrylate, butanediol diacrylate, neopentyl glycol diacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate, dipropylene glycol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacylate and the methacrylate analogues. The copolymer (II) may be up to 70% by weight of the solution of copolymer (II) and solvent. Preferably the copolymer (II) represents up to 50% by weight of the solution.
In the process of the present invention we include the reaction where Y is an amine in the term xe2x80x9cesterificationxe2x80x9d
After, taking the copolymer (II) up into solution, the moiety that carries both a reactive group Q capable of undergoing esterification with the group Y and also a (meth)acrylate group so as to form groups R is added. Examples of such moieties include (meth)acrylic acid, hydroxy ethyl (meth)acrylate, hydroxy propyl (meth)acrylate, hydroxy butyl (meth)acrylate and amino ethyl (meth)acrylate.
An essential feature of the process of the present invention relates to the use of a reagent of the general formula (II)
Z1xe2x88x92N=C=Nxe2x88x92Z2xe2x80x83xe2x80x83(III)
wherein Z1 and Z2 are end groups which are capable of assisting in the solubilisation of the reagent by the solvent, such as optionally substituted C3 to C16 alkyl, C5 to C6 cycloaliphatic, phenyl or tolyl.
Unlike other esterifications which use catalysts such as p-toluene sulphonic acid, the reagent (III) promotes the esterification by removing the water of esterification. Consequently, unlike other esterifications, the esterification of the present invention is not equilibrium limited and highly quantitative yields can be obtained. Preferred reagents of general formula (III) include 1,3-diisopropyl carbodiimide, 1,3-dicyclohexylcarbodiimide and 2,2,6,6 tetraisopropyl diphenyl carbodiimide. Particularly preferred are those reagents of general formula (III) which after participating in the esterification tend to become insoluble and can be separated from the product by simple filtration, e.g. where the end groups are cyclohexyl.
Preferably the esterification is conducted in the presence of a base such as triethyl amine, tripropyl amine, tributyl amine, pyridine and dimethyl amino pyridine.
Another advantage of the process of the present invention is that the esterification can be conducted at relatively low temperatures, e.g. less than 40xc2x0 C., preferably less than 30xc2x0 C. and particularly from 0 to 25xc2x0 C. This is a valuable characteristic where the copolymer (I) can be cross-linked by heat or where it is formed in the presence of an oligomer and/or reactive monomer which is heat sensitive.
The copolymer (I) may be used in a wide variety of curable compositions such as conventional curable inks, overprint varnishes, photoresists, adhesives and flexographic printing plates.
When used in conventional curable inks; such inks include plasticisers, dyes, pigments, curing initiators, e.g. photoinitiators for UV curable inks or thermal initiators for heat sensitive inks. Typically, the amount of copolymer (I) used in the curable ink will be from 1 to 50% by weight and preferably from 2 to 25% by weight of the ink. Consequently, where the copolymer (I) is formed in the presence of reactive monomers the resulting product contains a higher concentration of copolymer (I) than is usually used in a curable ink and thus may require dilution. Similar concentrations of copolymer (I) may be used in respect of overprint varnishes, which by their nature do not contain dyes or pigments.
The present invention is further illustrated by reference to the following examples.